A variety of security devices have been proposed in the past to prevent security documents from being counterfeited or fraudulently produced. A particularly useful security device is one which is readily verifiable by a user but which is difficult to produce. An example of such a security device is a “see-through” feature in which complementary images are provided on each side of a document precisely registered relative to one another such that when the document is held up to the light, the image on the back will fit exactly into spaces within the image on the front. For example, each image could comprise a series of coloured segments, segments on one side of the sheet fitting within the spaces between the segments on the other. Printing of these images is normally carried out with specialised lithographic presses which allow simultaneous front and back printing during one printing run.
See-through features have four modes of visual inspection—the image on the first side of the document viewed in reflected light, the image on the other side of the document viewed in reflected light, the composite image viewed by transmitted light as viewed from the first side and with the image on that side predominating, and finally the composite image as viewed on the other side of the sheet with the image on that side predominating. On transmissive viewing of see-through features the image on the opposite side of the document is seen to be in register in a genuine document.
Some known security devices comprise a substrate having a viewing region which is provided on one side with first indicia and on the other side with second indicia overlying the first indicia. The substrate may carry an obscuring material aligned with the second indicia so as to prevent the second indicia from being viewed from the one side of the substrate under reflected radiation.
Other known security features include patterns aligned on the front and back surfaces of a document. The document may be sufficiently transparent to allow see-through of the partial image on the back of the document to be superimposed on the partial image on the front of the document to form a complete image if the patterns are properly aligned.
Security devices which exploit subtractive colour mixing are also known. When such a device is viewed in reflection on either side, the contrast between the two colours enables the two patterns on that side to be distinguished. However, when the device is viewed in transmission, overlapping, different colours subtract respective wavelengths or wavelength bands from the incident light such that the resultant light transmitted through both sets of overlapping patterns has substantially the same colour and thus it is no longer possible to distinguish between the individual patterns.
Although security devices in the prior art perform satisfactorily, they have a number of disadvantages. In particular, the patterns of the conventional see-through features must be precisely registered relative to one another for images to be recognisable. Complete images are not recognisable if the patterns provided on either side of the device are misaligned.
The present invention is intended to avoid the drawbacks of the security features in the prior art and to provide improved features which exhibit novel surprising effects to prevent counterfeiting.